pH Measurement

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Hanna Instruments offers both single parameter and multiparameter instruments in order to meet a variety of testing requirements.

Hanna Instruments offers both single parameter and multiparameter instruments in order to meet a variety of testing requirements.

Using Single Parameter

Hanna single parameter instruments offer simple, accurate and efficient measurement focused on, as the name implies, a single parameter.They are well suited to focused testing where one parameter must be tested quickly and easily.They are generally simple to operate and can be used by non-technical users.

Using Multiparameter

The advantage of Hanna multiparameter instruments is that a user can choose a single meter with the ability to measure multiple parameters .

Multiparameter instruments offer different operating solutions well suited to meeting multiple requirements and are available in two primary configurations:

  1. Multiparameter meters that can measure two or three parameters, but only one parameter at a time.
  2. Multiparameter meters that offer two or three parameters measured simultaneously—useful on experimental and research applications where the influence between the parameters is an important factor. Multiple inputs provide the capability for simultaneous measurement.

pH Measurement Input

Hanna meters generally come in two different electrode connection types: BNC or DIN.

BNC Connector: A BNC (Bayonet Neil-Concelman) is a common connector used for coaxial cable devices.A BNC connection is generally used for combined electrodes and half-cell electrodes that require a separate reference probe and separate reference input.

DIN Connector: A DIN (Deutches Institut für Normung) is a circular connector.It is used to connect amplified pH measurement electrodes.Electrodes utilizing a DIN connector feature a built-in temperature sensor.

Temperature Input

Temperature has an effect on pH measurements.As such, temperature compensation is required for accurate measurements.Temperature compensation can be obtained in three ways:

  1. A separate probe specifically for measuring temperature
  2. A probe with a temperature sensor built-in.
  3. Manual adjustment for temperature

If a temperature input is not present, many instruments still offer the ability to manually adjust the temperature according to an external temperature reference.

pH measurement Temperature Compensation

pH readings must be temperature compensated in order to obtain accurate results.The source of temperature measurement can be from a temperature sensor or from a trimmer that is manually adjusted.In either case, the instrument is adjusting the pH reading to compensate for changes in the pH sensor.Compensation in pH provides the actual pH at the temperature of measurement.

mV Reading

Hanna meters with an mV feature offer the ability to read the mV potential from a pH, ORP, or ISE electrode.The relative mV allows the user to offset mV difference generated from sensors or references.

pH/ISE Calibration

pH calibration should be performed daily or every time a new lot of readings is started.Any errors during calibration will affect all the readings until a new calibration is performed.Errors during the calibration process can be eliminated if standard calibration procedures are followed.

Hanna recommends the following standard calibration procedure:

  • Clean and activate the electrode before the calibration.
  • Use fresh pH buffers and standards.
  • Rinse the electrode with purified water during the calibration process to avoid buffer contamination then a rinse in buffer or standard.
  • Wait for a stable reading before the calibration point is confirmed.
  • Temperature compensation of pH reading and pH buffers.

Calibration is a key component to ensuring accurate readings during pH measurement.With this in mind, Hanna supplies each of our pH instruments with a starter package of calibration solutions.

pH CAL Check™

Many instruments feature Hanna’s exclusive pH CAL Check technology.CAL Check is a diagnostics system that ensures accurate pH readings every time.By alerting users to potential problems during the calibration process, the CAL Check system eliminates erroneous readings due to dirty or faulty pH electrodes or contaminated pH buffer solutions during calibration.

During the calibration process, users are prompted with a step-by-step, on-screen tutorial.After calibration, the electrode is evaluated and the condition and response time is provided.Depending upon meter, this may be a graphic of GLP information.

Calibration Errors

Instruments utilizing Hanna’s CAL Check technology can evaluate an electrode during calibration and store a history of parameters that describe the quality of electrode to be compared from one calibration to another.During calibration, a very small degradation of these parameters is normal and can be expected.A big change in the parameters signifies an error in the calibration procedure, such as a dirty electrode.

pH Buffer Contamination

pH buffers can be contaminated during the calibration procedure by numerous factors such as introducing a contaminated probe, using old buffers, or by reusing buffers.These factors may cause inaccurate calibration and subsequent measurements.

Hanna’s CAL Check can often detect issues during calibration, giving warning messages to inform users about the identified issue.

Response Time of Electrodes

Another parameter that is evaluated during the calibration with certain meters that have CAL Check technology is the response time of an electrode.This is evaluated based on the amount of time necessary to reach stability when the electrode is immersed in a new buffer that has a difference in pH larger than 3 pH units from the old one.

Offset and Slope of pH Electrode

The offset and slope are the most important parameters that can describe the quality of an electrode.With Hanna’s CAL Check technology, the offset of the electrode can be evaluated using one point calibration.Offset is generally determined using a 7.01 pH buffer, however, using CAL Check allows the offset to be based on any calibration point.The acceptable range for offset is ±30 mV although a warning may be displayed.

A minimum of two calibration points is necessary to determine the slope.Slope can be evaluated between two calibration points and normally should fall within a range of 92% to 110%, where 100% is 59.16 mV/pH @ 25°C.

Calibration Points and pH buffers

The calibration of a pH electrode is normally performed using two points: 7 pH, and 4 or 10 pH.This is based on the assumption that the pH electrode is linear from 3 pH up to 10 pH.For the most accurate reading, Hanna recommends using a calibration point closest to the values received during normal measurement.

For a variety of applications and measuring points, many Hanna meters offer the ability to calibrate using more than two points.Many Hanna instruments offer 2, 3, or up to 5 calibration points for enhanced accuracy.pH buffers 1.68, 3.00, 4.01, 6.86, 7.01, 9.18, 10.01 and 12.45 cover the entire pH range.

During calibration, the recognized pH buffers are temperature compensated by the instrument in order to account for pH variation of buffers due to temperature.For example, a 10.01 pH buffer is 10.01 pH only @ 25°C.A table of temperature variation is printed on the label of each pH buffer.

Custom pH Buffers

Hanna has implemented the concept of custom pH buffers into many of its instruments.This permits the user to add an industry specific buffer for calibration.However, temperature compensation during calibration is not implemented because the temperature variation correlation is unknown.

Stability During Calibration

The stability of readings is important in order to avoid incorrect calibration.Based on this, the confirmation of a new calibration point is done only after stability is reached.Users are informed during all processes about the stability conditions, and any instability will restart the stability evaluation.The stability criteria during the calibration is more rigorous than during the measurement.This mode used in Hanna instrumentation avoids errors by confirmation of calibration points during unstable readings.This principle is respected in any type of calibration, manual or automatic.

Out of Calibration Range

This is an important feature during measurement and is considered Good Laboratory Practice (GLP).The measurement is considered more accurate.If the measurement reading is in a range far from the calibration points, the “out of calibration range” message is displayed.The measured value is shown and the user can accept it, but with the warning from the instrument related to possible inaccuracy.

Calibration Reminder

The calibration reminder, like “out of calibration range,” is a GLP warning message.Regularly scheduled calibrations are crucial for accurate and repeatable measurements.A warning reminder will be displayed when the sensor needs calibration.Measurements can still be used under the warning reminder.

Step-by-Step Calibration

In order to avoid errors during the calibration procedure, the meters display indicators that can be followed by the user for a successful calibration.If necessary, it is possible for the calibration steps to be performed in a different order by the user.

Additional Features

GLP and ISO standards require the traceability of operations.Hanna’s GLP document the quality of calibration, plus information to identify the instrument, operator, and the time at which calibration was performed.

Logging is a common feature for many instruments and can be used to record readings.Two working modes are available: log-on-demand and automatic or interval logging.With log-on-demand, measurements that are considered important can be saved with the press of the log button.With automatic or interval logging, the instrument saves all the readings according to a specified interval.Another logging mode is Auto-End logging or log on stability.

Many Hanna meters include graphic LCD’s with features such as tutorials, contextual help, multi-language support and icons and messages to guide the user through operation and calibration.

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